Acids and bases are used in a number of different processes in the refining, chemical, petrochemical, and pharmaceutical industries among others. Acids, bases, and salts may also be formed from syntheses, as reaction byproducts. It is often desired to eliminate these acids, bases or salts from the gas or liquid (or fluid) stream of interest. Elimination of the acid, base or salts from the fluid is traditionally accomplished by means of a scrubbing process, where a scrubbing liquid which is a separate phase from the fluid of interest is added, the scrubbing process may involve neutralization if an acid or base is involved. For example, a base in the form of a liquid is added to the fluid to neutralize the acids, and an acid in the form of a liquid is added to the fluid to neutralize the bases. If salts are present, water is used to scrub the salts out of the stream. Typically, an excess of the scrubbing component must be added to assure complete removal of acid, base or salt. The neutralization of the acid or base by the neutralizer results in salt byproducts being formed. Following the neutralization process, the excess neutralizer and salt byproducts must be removed.
Since the scrubbing liquid is a separate phase from the fluid of interest, the scrubbing is generally accomplished in columns where the scrubbing liquid is dispersed into the fluid of interest to facilitate the extraction. Depending on densities, one of the fluids will rise and the other will descend. For example, if the fluid of interest was a gas, gas bubbles rise up the column, contacting the liquid neutralizer. The excess neutralizer and reaction byproducts are subsequently removed at the bottom of the column. If the fluid of interest was a light hydrocarbon (specific gravity of 0.5), and the scrubbing fluid was water (specific gravity of 1.0), the water would be added on top and would descend down the column, whereas the hydrocarbon would be added at the bottom and would rise up the column. The efficacy of this system is related to the mass transfer efficiency between the two phases. This is directly related to the specific contact area (area per unit volume) that is available for mass transfer. To increase this area, many columns will typically use either structured or unstructured packing. The limited specific contact area possible necessitates increasing the size of the packing. The fluid stream, exiting the column will typically entrain with it, an aerosol or emulsion of the scrubbing liquid that may cause challenges downstream. Therefore, it becomes necessary to introduce high-efficiency aerosol-removal, or emulsion separation downstream. Typically, then, the mass transfer between the scrubbing liquid and the fluid of interest, and the complete separation of these fluids occur in two separate devices.
A disadvantage of the above conventional two-stage scrubbing process is associated with the capital costs for the hardware such as towers and reaction tanks.
Similarly, in the refining and other industries, gaseous hydrocarbon streams that contain a range of hydrocarbons are stripped of the heavier hydrocarbon components through absorption into absorption oil in an absorber column or an absorber stripped column.
The present invention provides a process for the removal of an unwanted component from a gas or liquid by introducing an extracting liquid to extract the unwanted component from this gas or liquid through an interaction between the extracting liquid and the unwanted component. In a preferred embodiment, the volume of extracting liquid can be generally the same as the volume of the component to be extracted. More specifically, this invention relates to the process of creating an aerosol or emulsion of an extractive liquid, capturing this aerosol or liquid on a high specific area microstructure to effect the extraction of the unwanted component and separation of the liquid phase within this microstructure. The extraction occurs from the fluid of interest to a scrubbing liquid phase that is either stably dispersed in the primary phase gas or a film on the porous medium. In the case of the removal of an acid, base or salts from a gas or liquid stream, the process involves creating an aerosol or emulsion or dispersion of a polar scrubbing liquid phase that is stably dispersed in the gas or liquid stream and forms a film on the porous medium. In the case of removing heavier hydrocarbons, the process involves creating an aerosol or dispersion of an extractive liquid that oleophilically interacts with the heavy hydrocarbons in the gas to form a “rich” oil phase that is stably dispersed in the light hydrocarbon gas stream and forms a film on the porous medium.
This stable dispersion, may be defined as a stable suspension of a discontinuous liquid phase within another continuous gas or liquid phase that is not separable by conventional gas/liquid separation technologies—such as filter-coalescers, residence time coalescers with mesh-pads or vane-packs, or filter separators, coalescing beds etc. For such stability, the discontinuous liquid phase consists of droplets in the 0.1-1-micron range for dispersion within the gas phase, with the larger droplet end of the spectrum possibly extending up to 10-micron range, and of droplets in the 0.1-10 micron range for dispersion in a continuous liquid phase with the larger droplet end of the spectrum possibly extending up to 100 microns. This stable dispersion is necessary to facilitate the first stage of the intimate mass-transfer between the primary and secondary phases. Following the dispersion, the second stage of the invention relates to then using a coalescer such as a porous medium to capture, coalesce, and separate the rich liquid in the form of droplets from the continuous gas or liquid. The film of rich liquid on the high surface area porous medium provides a secondary stage for extraction. In order for the porous medium to capture the droplets it must be constituted with fibers that are of such dimensions and interfacial properties as to be able to be “wetted-out” by the liquid, thus enabling it to capture these droplets. This typically requires the fibers to be of the order of magnitude of the droplets; in other words, the porous medium must consist of fibers that are at least in the 0.5-2-micron range. This invention then provides for the contact and separation of the extracting medium in a single device. A feature of the present invention is that the droplets to be created are a microdispersion, more specifically a “stable microemulsion” or a “stable aerosol”. More specifically, this microdispersion has been described as having droplets smaller than 10-micron, preferably smaller than 3-micron in size if we are dealing with liquids dispersed in liquids; and as having droplets preferably smaller than 1-micron if we are dealing with liquid droplets in a gas stream. There is a distinct difference between a dispersion and a microdispersion to those skilled in the art. Both dispersions and microdispersions (or aerosols or emulsions) consist of a liquid phases distributed within another fluid. A dispersion is understood by those skilled in the art to consist of droplets that are large enough to be separable by residence time, whereas an emulsion consists of droplets that are small enough to be stable for extended periods of time (at least >1 day). A dispersion can be separated by conventional filter-coalescers, residence time coalescers with mesh-pads or vane-packs (such as Bayley's Great Britain Patent No. 1,443,704 “perforate packing” of “knitted mesh fabric”). There are a number of reasons for this, one of which is that larger droplets have lower surface energy, making them energetically predisposed to coalesce into even larger droplets. A second reason is that droplets larger than the boundary layer on the coalescing surface, tend to inertially impact this surface, and given their predisposition to droplet coalescence, once they impact this surface these droplets are likely to coalesce into larger droplets. As the droplets get smaller, their surface energies increase, making them less energetically disposed to coalescence. Simultaneously, as the droplets approach the dimensions of the boundary layer, they tend to not inertially impact the fixed surface, but to flow around it with the fluid streamline. The combination of these factors makes the coalescence of droplets non-linearly more difficult as the droplet size declines.
Additionally, efficiency of extraction is a function of the specific surface area (surface area per unit volume). Table 1 below illustrates specific surface area as a function of droplet size. It is known to those skilled in the art that the higher the specific surface area, the greater the efficiency of separation. Table 1 also illustrates the typical settling time required as a function of droplet size for a specified system. The system described below has a very large specific gravity difference (hydrocarbon 0.4 g/cm3; and water 1.0 g/cm3, difference in specific gravity is 0.6 g/cm3) and the residence time, by Stokes' Law, is minimized compared to the systems with 0.01 g/cm3 difference discussed in the invention. Even in a system that should be easily separable, droplets smaller than 10 micron will take extremely long times to settle out. As the specific gravity difference diminishes, this settling time will linearly increase—for example, the same droplet will take twice as long to settle in a dispersion with specific gravity difference of 0.2 g/cm3 as it will in a dispersion with a specific gravity difference of 0.4 g/cm3.
TABLE 1Estimated Specific Surface Area and Separation RequirementsDroplet SizeSpecific Surface(micron)Area (m2/m3)Residence Time For Separation500600115seconds250120065seconds10030007minutes1030,00010hours1300,00045daysSystem consists of 5% water in a hydrocarbon stream wtth a specific gravity of 0.4 g/cm3 and a viscosity of 1 cP.